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Dipartimento di Fisica - Politecnico di Milano

Study of ultrafast electron dynamics in biomolecules by transient X-ray spectroscopy

This thesis work aim at the investigation of ultrafast dynamics in molecules by ultrafast spectroscopy with chemical sensitivity. A source of Soft-X attosecond pulse trains (150 - 550 eV) based on high-order harmonic generation in a micro-structured device will be exploited for Near-Edge X-Ray-Absorption Fine-Structure Spectroscopy. Ultrafast processes, such as heterocyclic ring opening and dissociation will be investigated in molecules relevant in biological processes.

Understanding attosecond-scale (1 as = 10-18 s) response of matter has huge implications in several fields of science since it discloses the way a system initially behaves upon external excitation. This is of exceptional importance in biological systems, where the photophysics of fundamental processes (light harvesting, DNA radiation damage and related carcinogenesis, vision mechanisms etc.) is mostly known on temporal scales longer than few tens of femtoseconds. On those scales "purely electronic" phenomena, i.e. the initial steps in the response of natural systems (photoionization, electron correlation effects, charge migration etc.) remain unresolved.
Applying ultrafast spectroscopy with chemical specificity on gas-phase molecules we will get a deep allows the investigation of attosecond phenomena in biorelevant molecules giving insight into several primary mechanisms, with a huge impact in Biology, Chemistry, and Molecular Physics.
Pioneering investigations recently revealed the occurrence of charge migration processes driven by electron coherence in photoexcited biomolecules. Those studies were based on the detection of charged particles emerging from molecular ionization and dissociation. Although providing new knowledge on the initial steps of photoexcitation, that approach still doesn't access the full picture of the biomolecule behavior. Additional information is provided by chemically-sensitive spectroscopy, such as time-resolved Near-Edge X-Ray-Absorption Fine-Structure Spectroscopy (NEXAFS). This powerful technique, available in our lab, is element-sensitive and is able to probe the unoccupied bound states of the target molecule, providing information on the local electronic environment.

The generation of attosecond pulses relies on high-order harmonic generation (HHG) process. High-order harmonics are generated when an intense laser pulse is focused on a gas medium: due to the strong nonlinear interaction, very high odd harmonics of the driving pulse optical frequency can be generated, leading to the production of a train of light bursts with attosecond duration in the XUV spectral region. Since the maximum photon energy in the harmonic spectrum increases as the square of the driving wavelength, longer driving wavelengths extend the HHG cutoff and allow the generation of IAPs beyond the water window (300 - 550 eV). In this regard, a real breakthrough was the development of tunable, intense and few-cycle optical parametric amplifiers (OPAs) in the mid-IR performed in our lab.

The realization of NEXAFS in biorelevant molecules enables the tracking in real time of the charge dynamics occurring in the molecules with a deep insight into the interplay between coherent electron motion and nuclear rearrangement. The element selectivity is also be crucial to disclose the functional groups involved in the dynamics.

Part of the activity is performed in collaboration with FERMI Free Electron Laser

This thesis work will be performed in the ERC-granted UDynI laboratory:


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